The Battle of the Bugs
Isn't it funny how even those of us quite accustomed to finding the odd flea traversing the tummy of a barn cat or Jack Russell react with the utmost horror to the sight of something crawling on our horses? We don't tend to give external parasites--creepy little critters like mites, lice, and ticks-a lot of consideration in our day-to-day horse management, but they can have just as much impact on our equines' health as the internal parasites (worms and bots) we're so much more used to dealing with.
Courtesy University of Ky./Dr. Fred Knapp
Ticks are responsible for carrying an alarming number of diseases.
External parasites need a little more press--and not only because they give us the willies. They can be responsible for a host of symptoms ranging from mild (skin itching or hair loss) to severe (spasms, seizures, and blood loss). But there's another serious consideration as well--many external parasites are disease carriers, and some of those diseases are deadly. So let's delve into the secret world of multi-legged crawlers and find out a little about how they live and how to combat them.
Know Thine Enemy
Although flying insects like warble flies and bot flies can be construed, in certain life stages, as external parasites of horses, for our purposes we'll be talking about the non-flying variety: mites, ticks, lice, fleas, and chiggers. All of these have certain traits in common--they're crawlers who jump on to horses from other "hosts" or from the environment (some ticks, for example, lurk in trees and drop down on unsuspecting horses and riders); they feed on skin, blood, or secretions; and they're small (ranging from about the size of a raisin down to microscopic). Not all are insects--mites and ticks, possessed of more than six legs, are categorized as arachnids (in the same family as spiders).
Most external parasites have life cycles that include several stages, and not all of those stages necessarily are spent living on a horse. In some cases, external parasites go through several hosts as they progress from egg to larva to nymph to adult; these varieties are usually a problem for horses only in warm weather. Others, who spend their entire lives buried in equine fur, can be troublesome even in winter or early spring.
Where can you find external parasites on a horse? They are found chiefly, in the region of the mane and tail, which are favorite spots for burrowing and munching on sebaceous secretions. But these critters also can make themselves at home in the belly hairs, on the underside of the neck, in the fetlock "feathers," and the inner thighs, as well as along the withers and even in the ears.
The signs of infestation vary from species to species, and a definitive diagnosis sometimes can be a challenge; even an experienced veterinarian might not be able to distinguish one type of parasite from another with the naked eye because of their smallness. Thus, you might have to resort to a skin scraping. This involves using a dull blade to collect dead surface skin cells and related debris, often from the base of the mane or root of the tail, then examining the collection under a microscope. Secretions, eggs, and insect parts observed there usually will lead to an identification of the perpetrator. Should your veterinarian not consider himself or herself an arthropod expert, you might have to seek an entomologist at your nearest university or agricultural extension service to put a name to your horse's free-loading guests. (If your skin scraping turns up no obvious signs of critter infestation, your veterinarian might want to rule out other causes by doing bacterial or fungal cultures, allergy tests, or even a skin biopsy.)
Here's a rundown of the most common external parasites found on horses:
Probably the most common of equine external parasites (especially in temperate climates), lice come in two varieties: chewing or biting lice (Bovicola equi and certain poultry lice), which feed on skin cells and sebaceous secretions, and sucking lice (Haemotopinus asini), which feed on blood.
If your horse has lice, it's likely he will have heavy dandruff and greasy skin. Blood-sucking lice tend to congregate in the mane and tail, along the head and neck, and on the inner thighs, where they might eventually cause bald spots with raw, red centers. If present in sufficient numbers, they can even cause anemia. Part the hair at the base of the mane, and you will see lice moving about at the hair roots, looking like small, dirty specks. You'll have to look closely, though, because even the adults average only one-tenth of an inch long.
Infestations of lice often are worst in the early spring, when accumulated dirt in the barn and tack room, plus an abundance of dander from horses shedding their winter coats, provide an ideal environment for them. Horses also can contract lice by picking them up from poultry--so getting rid of any chickens on the farm is a good first step if you discover lice on your horse. They cannot contract them from cattle, however; nor do equine lice transfer themselves to people. (Whew!) Thin, aged, stressed, or physically compromised horses seem to be more susceptible to lice, and they are frequently (although not always) the result of poor management and grooming.
These are almost-microscopic creatures that live practically everywhere-lurking in hay, straw, and grain. Barn swallows and sparrows that frequent haylofts are another common source. Papular eruptions (like small pimples) on the neck and withers are one sign of an infestation of mites; the bumps might or might not be itchy. Dandruff and hair loss, especially in the mane and tail, also are possible, especially with bird mites, which tend to be transitory.
At their worst, mites cause mange, a miserable skin condition that can include body-wide papules, patchy baldness, and moist, bloody crusts on the mane, tail, and forelock areas. There are three kinds of mange mites:
a) Sarcoptic mites--circular in shape, with short legs. They usually cause small skin nodules with hair loss, where eventually scabs develop as the mite burrows into the skin. They can be found on the head and neck, shoulders, flanks, and abdomen.
b) Psoroptic or scab mites--oval bodies with longer legs and segmented pedicels. These can be seen on the skin surface, where they cause the moist, gooey scabs and crusts described above. (You'll need to look closely to observe them--unlike lice, they generally aren't moving around.)
c) Chorioptic mites--the most common type, they resemble the psoroptic variety quite closely, but have unsegmented pedicels. They also can be seen on the skin, where they cause scaling, especially on legs with heavy hair growth--a condition that is sometimes called "Clydesdale itch."
Transmission of mites is made by casual contact, so an infested horse should be isolated from his neighbors and treated promptly (more on treatment options in a moment). It also might be necessary to dispose of any bedding or hay you suspect of harboring mites (since they also affect humans, you'll probably know which bales are suspicious!).
We don't tend to think of fleas as an equine problem, and generally, they aren't--but it is quite possible for horses to pick up a flea population from barn cats or from poultry. (Once again, chickens and horses probably are a poor mix.) The signs of flea infestation are very similar to those we'd expect from a dog or cat--local itching, skin inflammation and irritation, and possible anemia.
These actually are the larval form of various Trombidium species of mites, sometimes called harvest mites. They are microscopic, orange-colored bugs that can be identified in skin scrapings. Horses infested with chiggers suffer from small, itchy bumps, particularly on the head, chest, and neck, which result from the critters burrowing into the epidermal tissue. The itching might be so severe that the horse rubs large areas raw. Chiggers don't hang around--they tend to infest one horse's skin for about a week, then move on to another horse (which, of course, means they can go through an entire herd in the space of a summer). Since they are found in short grass, rotting wood, and hay, it can be difficult to control chiggers in the environment. So, if you have an established problem, you might be forced to keep horses inside while you treat the fields with insecticides.
Sudden, frantic summer itching of the mane and tail most often indicates an infestation of ticks. Although they only spend a brief part of their life cycle on the horse, these probably are the nastiest of the horse's external parasites, because ticks are responsible, or are suspected of being responsible, for carrying an alarming array of diseases, including Rocky Mountain spotted fever, encephalomyelitis (sleeping sickness), Colorado tick fever, tuleramia, Powassan encephalitis virus, anaplasmosis, EIA (swamp fever), tick paralysis, piroplasmosis, equine granulocytic ehrlichiosis (EGE), and Lyme disease.
Lyme disease is probably the biggest concern of horse owners at the moment. Caused by a spirochete called Borrelia burgdorferi, which is transmitted from animal to animal by ticks (white-footed mice and white-tailed deer being two identified hosts), Lyme disease was at first thought not to affect horses--but unfortunately, that has turned out to be untrue. In fact, the incidence of Lyme disease in equines is on the rise throughout the Northeastern United States, where the carrier ticks are most prevalent. A large percentage of exposed horses do not demonstrate any symptoms; but those who do might suffer fever, a subtle, shifting lameness (which often baffles handlers and veterinarians), swelling of the limbs, a reluctance to move (which can mimic "tying-up"), laminitis, inflamed eye tissues, lethargy, and skin and muscle hypersensitivity. If left untreated, horses can develop a blue-green discoloration of the cornea and yellowing of the eye's normal white sclera, as well as eventual hepatitis and/or nephritis and chronic arthritis. The disease also can cause abortion in broodmares and be transferred across the placenta to foals in utero.
Fortunately, if a veterinarian can sort through this confusing gamut of symptoms, Lyme disease can be treated successfully with systemic antibiotics such as penicillin and tetracycline. (A shifting lameness that responds to antibiotics is a classic Lyme disease tip-off, in fact.) A blood test usually will reveal the presence of the spirochete--which is good, because the evidence of the tick that transmitted the disease usually is long gone by the time the horse develops symptoms.
Another tick-borne disease of concern to horse-owners in the region of Northern California is equine granulocytic ehrlichiosis, which produces flu-like symptoms such as shaking and chills, and can lead (if left untreated) to fatal organ damage. Transmitted by the Western black-legged tick, EGE also can be suffered by humans, although the disease is not directly transmittable between horses and humans.
Ticks are a serious nuisance even without the risk of disease; they latch onto a horse's tissues with bulldog tenacity with their long, strong mouth parts, and easily can cause a horse to rub his tail bald with itchiness and frustration. Even long after the tick has drunk his fill of your horse's blood and dropped off, the itchiness lingers (it is often most intense after the tick has gone, in fact). If you part the tail hairs, you might see the ticks' calling card, bits of dry serum--little crusty granules--among the roots. A severe infestation of ticks can cause anemia, poor condition, and a fluid build-up in the abdomen. Tick bites can become infected, producing large, dark scabs with creamy yellow pus lurking underneath (these crusts should be removed and the area scrubbed with an anti-bacterial soap; wear gloves to ensure you don't spread infectious material to other horses).
Of the many types of ticks, one of the most insidious is the spinous ear tick (Otobius megnini). The larvae of these little nasties burrow deep into the ear canal and incubate there for one to seven months, creating a bizarre array of symptoms including sweating, pawing, head-tilting or shaking, ear shyness, inexplicable stiffness or reluctance to move (frequently mistaken for colic or for tying-up), and muscle spasms so severe they're confused with seizures. Summer and early fall are the most common times for horses to develop these symptoms. If left undetected, spinous ear ticks can cause debilitating nerve and membrane damage, even deafness. Fortunately, the damage usually is temporary and symptoms reverse once the ticks are removed from the ear. (Other varieties of parasites, including the Lone Star tick and Psoroptes mites, can sometimes be found in the ear canal.)
Dealing with external parasites requires a two-pronged approach. Not only must you exterminate them on your horses, you must find a way simultaneously to control them in the surrounding environment.
There are available products specifically formulated to kill lice, mites, and ticks; the best have both an insecticide and an insect growth regulating chemical that helps interrupt the bug's life cycle. The combination kills adults and inhibits egg hatching and the development of the larvae, a necessary step when you have a heavy infestation to deal with. Look for an insecticide formula that contains pyrethrins--this chemical, extracted from chrysanthemums, is biodegradable and has a wide safety margin, making it the bug killer of choice around horses. (It's also the dominant chemical used in most fly sprays.)
Controlling external parasites on your property might be the easy part. Getting rid of them on your horse could require daily applications of pesticides during the peak bug season, sometimes for an extended period. Too-frequent applications of some of these chemicals can be toxic. Because horses have relatively sensitive skin compared to other livestock, they are prone to reactions to solvents and other ingredients in pesticide formulations, ranging from burning to blistering or cracking of the skin. (Minimize the risk by using only products formulated and approved for use on horses, and by testing any new product on a small patch of your horse's skin; observe for 24 hours for possible reactions before using it elsewhere on your animal.)
It goes without saying, of course, that you should follow the package directions to the letter, paying particular attention to recommended dilution rates, time between treatments, and methods of disposal. In addition, avoid treating any horse which is sick, stressed, or overheated, and be careful not to contaminate feed or water troughs accidentally in the process of bumping off the bugs.
Even with the use of pesticides, it's important to keep a constant patrol against parasitic critters. Each time you groom your horse, keep an eye out for signs of infestation--dandruff, itchiness, bald spots, scabs, or bumps. To help control ticks, in particular, mow, clear, and burn tick-infested areas (thick brush and woodland) in the immediate area of your barn and pastures, and discourage other tick hosts, such as mice, by keeping grain locked securely away in rodent-proof containers.
If you must ride in a wooded area that's known to harbor ticks, you can discourage them from attaching to your horse by applying bug repellent all over, and oiling the mane and tail, belly, and lower legs with mineral oil or baby oil at least once a week. The oil will make it difficult for ticks to latch on. (Don't forget to protect yourself by wearing light-colored, long-sleeved clothing and repellents!) On your return, check your horse thoroughly, especially among the hairs at the root of the mane and tail, and continue to check him for at least a week afterward--ticks are small and easy to miss, although when they are engorged with blood, they can reach the size of a plump raisin. Since most tick-borne diseases are only transmitted when a tick has been attached for 24 hours or more, early detection is crucial to preventing infection.
If you do find a tick, be careful how you remove it--remember that it might be a vector of disease. Because a tick's strong jaw anchors it so firmly to its host, it is very easy to pull a tick apart and leave the head embedded in your poor horse's flesh, inviting infection. The best way to remove a tick is not with your fingers, but with a pair of tweezers or splinter forceps. Grasp the bug as close to your horse's skin as possible, and gently pull straight out; don't worry if the mouth parts are left in the flesh, as they generally come loose soon afterwards and rarely cause problems. (An alternate method, if you lack a pair of tweezers, is to swab the tick in both directions with a cloth soaked in baby oil or rubbing alcohol. This has the effect of suffocating the tick, and eventually it will let go of your horse and drop off--but it might take hours or even days to do so.) As much as you might be tempted, don't then drop the tick on the ground and stomp on it--if it is infectious, that's an ideal way to spread disease in every direction. Instead, drop the tick in a jar of rubbing alcohol. (You might want to take the critter to your veterinarian or agricultural extension agent for a positive identification.) Finally, apply an antiseptic cream to your horse at the bite location.
Finally, although the product is not actually licensed for this use in North America, many veterinarians feel that the deworming drug ivermectin (Zimectrin or Eqvalan by trade name) is effective against external parasites in horses, based on its success in this area in other species like dogs and cattle. For example, the drug is now being used in dogs to control both heartworm and fleas simultaneously. It's also suspected that ivermectin's newer cousin, moxidectin (Quest), will have the same properties.
Andrew Peregrine, BVMS, PhD, DVM (Hons.), of the University of Guelph's parasitology department, explains that ivermectin and moxidectin permeate the horse's tissues, eventually reaching the epidermal (skin) cells, where in sufficient concentrations they are toxic to the adult forms of most external parasites. "It depends somewhat on the feeding habits of the parasite," he says. "The more the bugs ingest, the more effective it is--so burrowing and sucking parasites who get past the skin surface will be exposed to an accumulation of the drug in the keratin layers. It's not an immediate effect."
Ivermectin and moxidectin are likely not effective against ticks, he adds, since ticks spend only a limited amount of time on the horse. Nor will these drugs prevent ticks from attaching themselves to your horse's flesh. Because neither ivermectin nor moxidectin is currently licensed in the United States or Canada for use against external parasites, it's best to consult your veterinarian concerning its use.
Still get the heebie-jeebies after reading all of this? Take heart. Equine external parasites aren't all that common. But if your horses should come home with some freeloaders, we have an impressive arsenal of defenses against these crawling critters. Used conscientiously, they can eradicate any infestation problem your horse might have (which is a better success rate than you're likely to have with the ubiquitous internal parasites). And a little vigilance should ensure that your horse stays vermin-free.
About the Author
Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.
POLL: Visits from the Vet